LTM8055 Quiescent Current

The LTM8055 data sheet lists quiescent current into Vin (tied to SVin) as 8/30 mA typical/maximum in DCM and 45/100 mA in FCM. 

Using the DC2017A eval board as is, I get numbers within range: with no load at 28V in and 12V out, the measured quiescent current into Vin is 4.3 mA DCM and 53 mA FCM. However, when I increase the output voltage the quiescent current increases dramatically beyond the specification in the data sheet. I changed R2 to 4.7k to set the output voltage at 26.8V. For the same conditions (28 Vin and no load) quiescent current jumped to 63 mA CCM (almost double the spec of 30 mA maximum) and 95 mA FCM (right at the abs max of 100 mA).

The only load on the output is the feedback network which dissipates ~7mW. The worst case light load efficiency from the figure "Efficiency vs Output Current (24Vout)" looks like roughly 30% so this translates to < 1mA or so at the input. Why is quiescent current so much higher than listed in the data sheet?

Parents
  • The spec in the data sheet is for the following condition:
    fsw = 300kHz (RT = 84.5k ohm)
    Vin = 12V
    Vout = 12V 

    Because the internal LDO is powered by VIN, the LDO’s power loss increases with VIN (increased voltage drop across LDO).  Also, an increase in switching frequency (increased current draw for gate drive) will increase the power loss in the internal LDO.  Also, input current will go up due to  increased inductor ripple current and increased switching losses resulting from the increased VIN/VOUT voltages.

Reply
  • The spec in the data sheet is for the following condition:
    fsw = 300kHz (RT = 84.5k ohm)
    Vin = 12V
    Vout = 12V 

    Because the internal LDO is powered by VIN, the LDO’s power loss increases with VIN (increased voltage drop across LDO).  Also, an increase in switching frequency (increased current draw for gate drive) will increase the power loss in the internal LDO.  Also, input current will go up due to  increased inductor ripple current and increased switching losses resulting from the increased VIN/VOUT voltages.

Children
No Data